Air/fuel feedback control systems are known in which fuel flow is corrected by a feedback variable derived from an exhaust gas oxygen sensor in an effort to maintain stoichiometric combustion. A two-state oxygen sensor is typically used in which the change in output state occurs at a reference air/fuel ratio. The system includes a three way catalytic converter having a peak efficiency window for optimal catalytic conversion of hydrocarbons, carbon monoxide, and nitrogen oxides. Under ideal conditions, the transition in output state of the sensor and the peak efficiency window of the catalytic converter both occur at the stoichiometric air/fuel ratio.
The inventors herein have recognized numerous problems with the above approaches. For example, the transition in exhaust gas oxygen sensor output states may not occur at stoichiometry for all sensors or over the life of any particular sensor. Furthermore, the peak efficiency window may not occur at stoichiometry for all catalytic converters. Accordingly, engine air/fuel ratio may not occur at the converter's peak efficiency window, thus resulting in less than optimal conversion of engine exhaust.